1. Field of the Invention
The invention relates to a synchronizer ring having a support body made from metal, comprising a conical friction surface, and having a friction layer of a material comprising carbon fibers applied to the friction surface. Moreover, the invention also pertains to a method of producing the synchronizer ring.
Synchronizer rings are used in a mechanical transmission, in particular in an automotive transmission, to match the relative velocities between gearwheel and transmission shaft which occur during a gear change to one another. The synchronization is in this case achieved by friction between the corresponding friction partners. In transmissions used for vehicles with powerful engines, the synchronizer rings are subject to increased wear, on account of the high frictional load. The same is also true of automatic transmissions, for example in a transmission of a commercial vehicle or in a transmission of a passenger automobile in which high gearshift forces are applied. It is customary for synchronizer rings to be made from metal, preferably from brass. Synchronizer rings of the type described in the introduction have been used for some time to increase the service life of highly stressed synchronizer rings and therefore also that of the transmission, with the additional friction layer protecting the support from wear.
2. Discussion of the Prior Art
U.S. Pat. No. 4,267,912 has disclosed a synchronizer ring with a friction layer which consists of a synthetic resin reinforced with cellulose fibers. However, under high stresses, this friction layer disadvantageously fails to withstand the thermal conditions which are then present. Also, the transmission oil cannot be sufficiently displaced, with the result that the friction partner floats, thereby reducing the friction.
To solve the problem, it is proposed in U.S. Pat. No. 4,878,282 that the friction layer be provided with grooves.
For this purpose, U.S. Pat. No. 5,615,758, U.S. Pat. No. 5,842,551 and U.S. Pat. No. 5,998,311 have described a structured carbon fiber fabric and its production. A fabric of this type is presented in U.S. Pat. No. 6,065,579 for use as a friction layer for a synchronizer ring. The weaving operation and the arrangement of the warp and fill yarns produce a pronounced three-dimensional structure with peaks and valleys. To be fixed, the fabric is impregnated with a resin and then conditioned. On account of the three-dimensional structure, the transmission oil can flow in and out for cooling purposes and can also easily be displaced in the event of the friction layer coming under pressure. The weaving operation, however, is highly complex and a friction layer of this type is expensive to produce.
U.S. Pat. No. 4,700,823 discloses the use of a friction layer made from a carbon fiber reinforced plastic for clutch or brake disks. The friction layer is produced by means of the relatively complex and therefore expensive chemical vapor deposition (CVD) process.
U.S. Pat. No. 5,851,588 discloses an open-mesh friction layer made from a carbon fiber reinforced plastic for large-area planar friction applications, such as clutch or brake; in this case, the friction layer has been produced by liquid pressure infiltration (LPI).
Another use of a carbon fiber reinforced friction layer produced by means of LPI for large-area, planar friction partners, as in the case of a clutch or a brake, is known from U.S. Pat. No. 5,952,249.
Disadvantageously, all the friction layers mentioned are compressed under the high compressive forces as occur in synchronizer rings on account of the conical friction surface, which is smaller than in the case of clutches or brakes. On account of the loss of axial reserve displacement, there is a risk of a shortened service life or of premature failure.